Weird deformation after exporting Rigify from Blender 2.8 to Unity
I had a similiar Problem with a Quadruped, where Parts of the Animation were completely broken: the Head had a wrong position while the ears were in the right position.
Apparently the Parent Child structure of the Deformation Bones is not suited for baking and does not always work correctly outside of Blender.

I solved this with the ORG Bones (Bottom Last Layer) as Deformation Bones instead of the actual Deformation Bones (Bottom Third Last Layer). 
Step by Step:
In Edit mode: 
- Select all Deformation bones in the Bottom Third Last Layer, go to Armature->Bone Settings->Deform (Hold Alt while clicking the checkbox, so it will untick the deform option for all of these bones)
-Select all Deformation bones in the Bottom Last Layer, go to Armature->Bone Settings->Deform (Hold Alt while clicking the checkbox, so it will tick the deform option for all of these bones)
-Rename the vertex groups so they have the Prefix ORG instead of DEF.